1. Field of the Invention
The invention relates to apparatus and methods for determining the nature of formations at the bottom of a wellbore for use in the oil and gas industry. More specifically the invention relates to the use of an ultraviolet fluorescence detector located in the drill string for determining the nature of the formations at the bottom of a wellbore to allow the drilling trajectory to be changed during drilling operations.
2. Background Art
When drilling it is important to know the nature of the formations located around the bottom of the well as it is drilled. By knowing the nature of the formations at the drill bit, is it possible to control the position of the drill bit and keep the well trajectory in the reservoir. Generally a well is drilled using a rotary drilling tool comprising a tubular drill string with a drill bit mounted at the end. A drilling fluid is supplied from the surface into the inside of the tubular drill string and pumped down to the drill bit where it is discharged into the wellbore, returning to the surface via the annular space between the drill string and the wellbore wall. The drilling fluid is used for the cooling and lubricating the drill bit and removing material cut away by the drill (cuttings) and for stabilizing the borehole during drilling.
Information regarding the drilling conditions can be obtained by measuring the properties of the drilling fluid, in particular the properties of the cuttings carried by the drilling fluid returning to the surface. One such property that can be measured is the presence of hydrocarbons in the drilling fluid. Detecting hydrocarbon fluorescence in the drilling fluid returning to the surface is known. Both U.S. Pat. No. 5,084,617 and U.S. Pat. No. 4,609,821 disclose detecting the presence of hydrocarbons in the drilling fluid cuttings. However, their analysis is performed once the drilling fluid with cuttings has returned to the surface. A problem with these methods is that, as the drilling depths can reach many hundreds of meters deep, there is a delay before the drilling fluid from the bottom of the well reaches the surface to be analysed and therefore the information the analysis it provides may no longer relate to the properties of the formations at the current location of the drill bit.
Being able to know as quickly as possible the nature of the formation at bottom of the wellbore is important so it is possible to alter the direction of the drill bit and enhance oil recovery by accurate well placement.
As the area where the drill bit can be several kilometers away from the surface, to determine what is happening at the area of the drill bit downhole techniques termed ‘measurement-while-drilling’ (MWD) and logging-while-drilling' (LWD) have been developed. These techniques involve placing sensors in the drill string to measure of variety of parameters which provide information on the nature of the wellbore and drilling process.
Currently measurements taken in known MWD and LWD techniques still take place some distance away from the drill bit location. Due to the mechanical vibrations and shocks of the drilling process and the lack of space near the drill bit it is difficult to place sensors close to the drill bit to preserve sufficient mechanical integrity. Therefore the measurements taken concern the formations at the level of the measuring devices and not the formations at the end of the drill bit. When the information regarding the properties of the formation reaches the surface, the drill bit can be some distance away from the formation where the measurements were taken.
U.S. Pat. No. 6,176,323 describes a drilling system with sensors to measure a variety of parameters. This document discloses measuring optical properties of the drilling fluid, using two sensors to determine whether native hydrocarbons are present in the drilling fluid. One sensor determines the properties of the drilling fluid in the drill string and a separate sensor determines the properties of the drilling fluid containing the cuttings returning to the surface.
GB2389380 describes a drilling system with sensors to determine the nature of the formations at the bottom of the well. The document discloses using at least two measuring means. One measuring means measures the properties of the drilling fluid inside the drill string and the second measuring means measures the properties of the drilling fluid outside the drill string, which is carrying the cuttings back up to the surface. The measuring means may measure electrical impedance, pH, nuclear density or electric voltage to determine the nature of the formations at the bottom of the well.
The invention proposes using an ultraviolet fluorescence detector that can receive drilling fluid from both inside and outside the drill string and can be located close to the drill bit. If the measurement can be taken close to the drill bit, information on the nature of the drill bit location can be received quickly and its path adjusted if necessary.